Generally, a wind turbine generator includes a turbine that has a rotor that includes a rotatable hub assembly having multiple blades. The blades transform mechanical wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators are generally, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbine generators also exist. The rotor, generator, gearbox and other components are typically mounted within a housing, or nacelle, that is positioned on top of a base that may be a truss or tubular tower. In some instances, one or more wind turbines that are situated relatively close together geographically can form a wind park or wind farm.
Some wind turbine generator configurations include doubly fed induction generators (DFIGs). Such configurations may also include power converters that are used to transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid connection. Moreover, such converters, in conjunction with the DFIG, also transmit electric power between the utility grid and the generator as well as transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid connection. DFIGs are used in wind turbines to permit variable-speed operation with minimum power-electronic power rating. These machines operate at speeds below synchronous (sub-synchronous) at low power, and at speeds above synchronous (super-synchronous) at high power. These wind turbines are connected to power grids, often operating in parallel with many other turbines on the same electrical collector system. The power grids can have many types of disturbances, some of which result in high-voltage conditions on the grid and on the wind turbine electrical systems. These disturbances can include: (1) remote events that can cause the voltage on the entire high-voltage grid to increase well above normal with gradual reduction back to normal; (2) local grid faults that can cause voltage at the wind turbines to be depressed, followed by sudden removal of the faulted circuit element. The sudden removal my cause an overshoot in voltage in a wind park until the wind turbines react to the new grid condition and regain control to bring the turbine back to normal operation into the portion of the grid that remains after fault clearing; or (3) local grid faults that, upon clearing, leave the wind plant with no remaining connection to the grid, but still with the wind turbines connected to the cables and lines of the wind plant and possibly a portion of the transmission grid. This can be considered an “islanded” condition for the wind park that is characterized by potentially significant deviations in voltage and frequency. This condition is not to be confused with other usages of the term “islanding”, where the intent is to ensure safety of maintenance personnel.
Each of the above-described events poses a potential for damage to the wind turbine electrical system due to high voltages within that system that exceed equipment capability. It is desirable for the wind turbine to ride through the grid events, both low-voltage and high-voltage, when the grid remains partially intact after clearing the grid fault. When the grid becomes open-circuited after clearing the fault, then it is desirable that the wind turbines continue operating without damage and eventually shut down based on inability to transfer power. In the latter situation, there is usually no time requirement for shutdown when the turbine is part of a wind park connected to a transmission grid. For distribution applications, local codes and regulations may require shutdown within a specified time, typically several seconds.
Accordingly, an improved system and/or method that respond to a high-voltage grid event on an electrical system connected with one or more DFIGs would be welcomed in the technology.